Method of preparing stainless-steel wire for drawing and forming



United States Patent METHOD or PREPARING STAINLESS-STEEL WIRE FOR DRAWING AND FORMING Joseph C. Grass'and Anton C. Zupansic, Wankegan, Ill.,

- assignorsto United States Steel Corporation, a corporation of New Jersey No Drawing. Application March 5, 1956 Serial No. 572,944

.1 Claim. 01. ms-6.24

the wire heretofore to lubricate the reducing or forming dies has failed to give satisfactory results. Galling of the dies occurs frequently; formed-products such as helical springs cannot be kept within established tolerances for length and diameter; the dies show excessive wear andthe'product exhibits objectionable roughness. We have invented amethod of treating process wire so as to form thereon a coating effective to overcome these difiiculties.

Our invention is based on the discovery that conventional processing of stainless-steel wire leaves the surface thereof so that it will not acquire and retain a coating which will satisfactorily lubricate drawing and forming dies. Our invention, therefore, includes a novel activating treatment followed by the formation on the Wire of a coating of novel composition which fulfills all requirements of a lubricant satisfactory for drawing and forming the wire. More particularly, our invention involves a treatment supplemental to the conventional processing which converts the surface of the wire from passivated to activated condition and produces a coating directly thereon by chemical reaction.

The conventional practice in making stainless-steel wire is to immerse bundles of hot-rolled or annealed rod or wire in molten sodium hydride to reduce the scale coating. The bundles are then quenched in water and rinsed under water sprays. The bundles are next immersed in a sulphuric-acid bath which causes removal of the reduced scale. The bundles are again rinsed and then immersed in a bath of dilute nitric and hydrofluoric acids to remove the smut produced by the sulphuric-acid bath. As a result of this treatment, the surface of the wire is left bright but passivated. The bundles are then rinsed and immersed in a hot dilute sulphuric-acid bath for a few minutes to break down or activate the passivated surface after which they are removed, dried, coated with a water suspension of lime and again dried in a flash baker. The rod or wire is then drawn to finished size and formed.

The method outlined above leaves the wire or rod deeply macro-etched prior to final drawing. This condition is not favorable to easy drawing or the formation of a smooth surface on the finished product. Our invention is a modification of the conventional process which produces a uniform fine-grained micro-etch on the wire surface and a coating which is particularly favorable to drawing and forming. More specifically, we add to the hot sulphuric-acid bath an accelerator containing sulphur which is released in colloidal form in an acid bath and tends to deposit on the wire and cling thereto upon removal from the acid. Then after lime-coating the bundles and drying them in a flash baker, we subject them to prolonged baking for several hours at a temperature of "ice from 800 to 1000 F. in an oxidizing atmosphere during which time the, sulphur combines with the base metal of the wire to form metallic sulphides. After this prolonged baking the bundles are again lime-coated and dried in a flash baker .whereupon they are ready forfinal drawing and forming. i

For a complete understanding of our invention, .it shouldbe noted that we follow the known practice up to the immersion of the bundles in a hot sulphuric-acid bath. This bath contains from 14 to 20% acid by weight and is maintained at a temperature between 130 and 185 F. We modify this bath by adding thereto an accelerator containing a compound which, when in'contact with acid, releases sulphur, such as sodium thiosulphate. One example thereof is the product known as Promat N0. 7073, made by Poor & Company, which is understood to be disclosed in Chester et al. Patent No. 2,692,187. The product contains from to 40% sodium thiosulphate and is added to the dilute sulphuric-acid bath in the ratio of one gallon of Promat to 80 gallons of the bath, giving a thiosulphate concentration of between .35 and 5% and preferably about 0.4%. The bundles are immersed in the modified pickling bath for from .20 minutes to 1 hour. The thiosulphate acts as an ac celerator in the acid bath by releasing free colloidal sulphur, The sulphur iactsas adepolarizer b'y combining with thehydrogen film accumulating on the surfaces of the immersed metal. In addition to accelerating the pickling action, the Promat produces a uniform finegrained micro-etch on the surface of the metal. This type of etched surface makes it possible to apply a more uniform and adherent lime coating preparatory to final drawing and results in a smoother finish on the drawn product. The modified pickling bath must also contain sufiicient metal sulphate to give it a density corresponding to a Baum reading between 15 and 25. Thus, if starting with fresh acid, it may be necessary to add iron until the Baum reading increases to at least 15.

The bundles when removed from the hot acid solution containing Promat are preferably allowed to dry in the air without rinsing. They may, however, after being removed from the hot acid solution containing Promat be given a water rinse prior to further treatment or they may be further treated without the benefit of either the rinsing or drying treatments. The adhering acid solution forms ferrous sulphate (FeSO -7H O) and probably also sulphates of chromium and nickel in smaller amounts. In any event, elemental sulphur is present on the dried surface of the wire or rod. When dried, the bundles are given a light-to-medium lime coating by immersion in a water suspension of lime. They are then dried in a flask baker and placed in a rotary-hearth furnace heated by an open flame from combustion of natural gas and baked for about four hours at a temperature from 800 to 1000 F. in an oxidizing atmosphere with an oxygen content of at least 1 or 2%. It is believed that the lime combines with the ferrous salt on the surface of the wire to form ferrous hydroxide (Fe(OH) and calcium sulphate (CaSO -2H O). The ferrous hydroxide is undoubtedly oxidized to hydrous ferric oxide (Fezog'llHgo) during the four-hour baking treatment at 900 F. The calcium sulphate will be dehydrated by the baking treatment and becomes anhydrous calcium sulphate (CaSO while some of the lime (Ca(OH) which does not combine with the ferrous sulphate will become calcium carbonate (CaCO because of the presence of carbon dioxide (CO in the furnace atmosphere. The lime (Ca(OH) which does not combine with the ferrous salts or the carbon dioxide of the furnace atmosphere will of course remain on the baked wire as Ca(OH) The final coating as developed by the. described practice conii ii f 'ii hi s f nn hr miu li i kel; l l 'i s ferric oxide (Fe Q -nH O), lime (Gamin and'probably calcium carbonate (CaQO and calcium sulphate a qa aa Wire treated as described may be easily. drawn to size it and then formed into articles such as springs by coiling or curlingdies. It also works well under cold-heading 59 I n case, h c a ser es are ra n forming lubricant which prevents excessive die wear and leaves a smooth surface on the finished article. Products such as springs, furthermore," may easily be made within permitted tolerances as to length and diameter with wire coated by our process, The'coating is dense, adherent and flexible so it does not crack, chip or flake off. 7

It is believed that the success of our invention is dependent upon a combination of factors including the uniform mirco-etched activated surfaceof the wire, the elemental sulphur and ferrous sulphate deposited thereon by the sulphuric acid and Promat solution, and the transformation of the elemental sulphur to metallic sulphides and the ferrous hydroxide to hydrous ferric oxide by the 800 to 1000 F. baking treatment in an oxidizing atmosphere. It is generally known that metallic sulphides are very satisfactory as lubricating films and a tight uniform non-abrasive oxide is also advantageous 4 v v for the cold drawing of carbon and stainless-steel wire. The coating which we obtain consists of a combination of the metallic sulphides ajid non-abrasive oxide films. The lime (Ca(OH) applied prior to the baking treatment combines with the ferrous sulphate to form the ferrous hydroxide whic'h,'as'. explained herein, is transformed during. the baking treatment to. the non-abrasive hydrous ferric oxide. i i i Although we have disclosed herein the preferred practice of our invention, we intend to cover. as "v'velk'any change or modification thereinwhich may be made without departing from the spirit and scope or the invention.

We claim:

A method of preparing stainless-steel wire for drawing and forming which consists in immersing the Wire for from 20 to minutes in a-dilute sulphuric-acid bath at a temperature between and F. containing an activating addition of between 3i and 5% sodium thiosulphate, then immersing: the wire in a water su'sp'ension of lime and baking for several hours at a temperatu re between 800 and 10 009 E, in an oxidizing, atmosphere, thereby forming on the wire a tough, ad fitent flexible coating composed at least in part offlime; metallic sulphide and hydrous ferric oxide. 7

References Cited in the file ofthis patent UNITED STATES 

